Aircraft: SR22
Phase of flight: Cruise
Anomaly: Aircraft Equipment Problem Critical; Aircraft Equipment Problem Less Severe

I was pilot in command and my passenger was a private-certificated; instrument-rated co-owner of the incident aircraft. We were on an IFR flight plan; the first leg of a multi-day trip. We were established in cruise at 15;000 feet and ahead of us at our altitude was a line of clouds that presented an icing risk; corroborated with a NEXRAD SLD risk; so I requested and received a climb to 17;000 from center. Before commencing the climb; while still programming the new altitude into the flight director; the engine suddenly began noticeably shaking and running rougher. I immediately responded by reducing power and telling center to 'standby' in response to the altitude change. I disabled the autopilot and began a left turn to where I had a general conception of the Central Valley (and flatter terrain) to be. Center again asked to confirm the altitude change; and I told them we were troubleshooting an engine problem. Center offered two airports; ZZZ more or less below us and ZZZ1.I was concentrating on how to react to the engine roughness; and eventually with my passenger's prompting; checked all major fuel-injected piston memory items: mixture rich; fuel pump on; switch tanks. This had no effect. Airspeed was decreasing and I had commenced a controlled 500-fpm descent from our cruising altitude. The engine roughness was manageable below about 30% power.Center meanwhile continued to offer up information about ZZZ (runway length; lighting; etc.) while I tried to focus on what to do about the engine. After conversing with Center; and perceiving the risks of attempting a landing at the smaller and more isolated ZZZ to outweigh the risk of flying to ZZZ1; I informed Center of our intention to divert to ZZZ1. Center gave me a heading. They offered to hand us off to approach but I was too task-saturated to accept the handoff. Shortly thereafter reception with Center became weak so I requested and received the handoff.After being handed off; I informed approach of our intention to maintain our slow descent and circle overhead the runway to lose altitude within gliding distance. They advised me that radar reception would be limited directly overhead so I opted to circle southeast of the runway in a position to enter the pattern. I was given clearance for a left traffic pattern for ZZZ1At this time; I finally had the presence of mind to call for the checklist; which my passenger began reading off. When trying the left magneto; the engine shut down; so I quickly returned the magneto switch to BOTH. Remembering that not 3 months ago the right magneto had failed on the ground in a similar way; I was now fairly certain that the left magneto had failed.I performed a descent checklist and had my passenger help prepare for the landing by getting the ATIS and finding an FBO. The circling descent was wide and left us well out of gliding distance from the airport; which my passenger objected to; so I corrected. We flew a high base to final and landed without incident.A later inspection revealed that the left magneto's main gear had been destroyed; exactly the failure that plagued the right magneto 3 months prior (on the ground).Factors affecting the quality of human performanceFactor: I failed to consider moving the magneto to the RIGHT position so that we could potentially get more power from the engine without excess vibrations.Root cause: Failure to complete the Engine Partial Power Loss checklist due to the 'scare factor' of the engine quitting when in the LEFT position.Contributing issues: A lack of critical thinking about the checklist's purposeFactor: I was slow to commence the Engine Partial Power Loss checklist.Root cause: I was having trouble concentrating on; and organizing my approach to; the emergency tasks.Contributing issues: A lack of familiarity with engine power loss scenarios; and interruptions in my train of thought in the course of help provided by the controller and my passenger. The controller and my passenger provided an abundance of unprompted help but I was unable to properly prioritize the radios lower than running the checklist.Factor: I circled outside of gliding range during my descent.Root cause: I lost the 'emergency mindset' once I believed that the failure was identical to that of the right magneto 3 months prior; and the situation became familiar to me.Factor: Initial human response to the engine power loss was slow.Root cause: Startle factor SR22 pilot reported a rough running engine at 15;000 feet and elected to divert to a suitable airport. During descent it was suspected that the left magneto had failed.